We are studying the involvement of free radical reactions in biological systems. There are now numerous examples of systems in which the evidence implicating radicals and radical reactions is very strong. In many of these systems, mechanistic data on the fundamental bio-organic free radical chemistry is not available. At present, we are studying three types of systems: l. Thiols and thiyl radicals as hydrogen transfer species. 2. Photolysis of thiols in solution. 3. Electron-transfer reactions which produce radicals and which involve species that are implicated in biological systems.